The present invention relates to a standby a.c. power supply having an inverter that will operate from either of two levels of d.c. voltage.
Uninterruptible power systems, or UPS, are available which are based on several different topologies.
For the most sensitive loads, the full time inverter or "true UPS" approach is best. In this topology, an inverter supplies a.c. to the load on a continuous basis. When the a.c. power line is available, the d.c. power required by the inverter is supplied from the a.c. line (via rectifier/battery charger). When the a.c. line is not available, the d.c. power required by the inverter is supplied by batteries. These batteries are recharged by the rectifier/charger when the a.c. line is again available. Since the inverter always supplies power to the load, there is absolutely no interruption or disturbance of the a.c. output when the power line fails or returns.
The disadvantages to this approach are cost and efficiency. Normally, all of the power required by the load goes through two power conversions: from a.c. to d.c. (via rectifier/charger) and then from d.c. back to a.c. (via the inverter). The advantage is that there is absolutely no disturbance in the output, and the output frequency is independent of the input. This is a particular advantage where the load is frequency sensitive and the input power is unstable (as in some third world countries or when operating off of an emergency generator or other such source). This also allows the UPS to be used as a frequency converter, such as when a 60 Hz load is to be operated on a 50 Hz power system. Also, no switching is required to disconnect the input to the UPS from the failed line, as there is no possibility of the inverter feeding power back to the input.
One of the reasons for poor efficiency in the true UPS in smaller sizes is the use of low voltage batteries. In a small system, the battery voltage will generally be in the 12 to 36 volt range. This is true for primarily economic reasons. Batteries are made up of individual cells, connected in series. A typical lead acid battery cell has a nominal voltage of 2 volts, thus a 12 volt battery has six cells, a 24 volts battery has 12 cells, and so on. For a given amount of energy storage, one can use relatively few large cells (low voltage, high current), or a greater quantity of smaller cells (higher voltage, lower current). At the relatively low power levels of interest here, the use of relatively fewer, larger cells is much less expensive than large number of smaller cells. Unfortunately, power conversion at low voltage and high current is much less efficient than power conversion at higher voltage and lower current. Semiconductor conduction losses of 1.0 volt, for example, are very significant on a 12 volt system, but much less so on a 250 volt system.
From the UPS designer's viewpoint, it is desireable to use a d.c. voltage in the 100-200 volt range. From a cost viewpoint, this is not practical in small systems because of the cost of the batteries.